at this goal, I don't know how to go further, and I would like to know about the hypothesis H1: l3 [= l1 ++ l2 can it rewrite to : l3 [= l1 /\ l3 [= l2? I do not find any proof about this case in the Coq's library (List).

Could you please help me? do I lack something in my lemma? and is it provable? Thank you very much.